The hardness came back into his eyes and he looked away.
It is an ironblack, opaque mineral, with metallic lustre; hardness about 6, sp. gr.
Then her attention was called to the hardness of the one ball and the softness of the other, and she learned SOFT and HARD.
You get used to knowing the different types of hardness and thickness.
A greater hardness is due to the presence of earthy impurities.